Pilot

ABSTRACT

A pilot ( 10 ) is shown for igniting a combustible fluid stream. The pilot comprises an inlet ( 12 ), for receiving a fuel gas ( 14 ), including a fuel and air/oxygen mixture, into the pilot. Ignition means ( 16 ), for example a spark igniter or an ignition electrode, are provided for igniting the fuel gas ( 14 ). A generally annular conduit ( 18 ) is open along an annular extent forming a circumferential aperture ( 20 ). The conduit ( 18 ) conveys fuel gas from the inlet to the circumferential aperture. When the fuel gas flowing out of the circumferential aperture ( 20 ) is ignited a continuous annular pilot flame is generated radially of the conduit.

The present invention relates to a pilot for igniting a combustiblefluid stream and to burners comprising such a pilot.

Burners are known for the combustive destruction of noxious substancesin a fluid stream. The combustion can be performed in a combustionchamber by an open flame arrangement or a radiant burning arrangement.

In an open flame burner 50 as shown partially in FIG. 6, a fluid stream52 is introduced through an inlet 54 of a combustion chamber 56 as amixture with a fuel gas 58. The mixture is ignited by a pilot flame 60of a pilot 62 and burns as an open flame 64 combusting the noxioussubstances. The pilot 62 comprises an injection tube 66 having an endproximate the inlet of the combustion chamber for introducing a fuel gaswhich is ignited by igniter 68 and which in turn ignites the mixture asit is introduced to the combustion chamber through the inlet 54.

A number of problems exist with the burner 50. The pilot flame 60 islocated at only one position relative to the flame 64 and thereforeburning of the flame is more readily achieved in the region close to thepilot flame and may not be achieved, or not fully achieved, at a regionremote from the pilot flame. Consequently, mixture entering thecombustion chamber may not be completely combusted resulting in noxioussubstances being exhausted from the burner. Further, the injection tubeof the pilot is typically relatively narrow and therefore the fuel gasis injected through the tube in a narrow concentrated jet which has apropensity to disrupt the flame 64 causing incomplete combustion of themixture. If more than one inlet is provided (not shown in FIG. 6), forinstance for introducing a fluid stream from more than one source ofnoxious substances, a pilot is required for each of the inlets causingadditional expense in addition to the problems identified above.

In a radiant burner 70 as shown in FIG. 7, a combustion chamber 72 isformed by a surrounding porous generally cylindrical wall 74. Fuel gas76 is introduced to an outer chamber 78 through inlet 80 and passesthrough the porous wall 74. A pilot 82 produces a pilot flame 84 forigniting fuel gas at the inner surface of the wall producing a hightemperature reaction zone 86. A weir arrangement 88 produces a weir ofcold liquid 90 (typically water) for dissolving constituents of thecombusted fluid stream and for washing away particulate matter. The coldliquid also cools fluid exhausted from the burner so that it can beconveniently disposed. A fluid stream 92 containing at least one noxioussubstance is introduced into the combustion chamber 72 through inlet 94and is combusted by contact with the gases from the high temperaturereaction zone 86 near the surface of the wall 74. Combustion is alsoachieved by the heat which is generated and reflected from an opposingsurface of the wall 74. A radiant burner is shown in more detail in EP0694735.

The burner 70 suffers from a number of problems. First, the flame 86when fully ignited is cylindrical and as the pilot flame 84 is locatedat only one position the high temperature reaction zone 86 ignition isassured in this region. The flame 86 may not burn or may not burn fullyat areas of the wall 74 which are remote from the pilot. Areas of thewall surface at which a high temperature reaction zone is not maintainedmay be susceptible to depositing of particulates which causes damage tothe porous wall. Further, the proximity of the cooling column cools thesurface of the wall 74 particularly at the lower part of the wall. Also,heat is not reflected from the cooling water which further acts to coolthe combustion chamber 72. Consequently, complete combustion of thefluid stream 92 may not be achieved along with incomplete combustion ofthe fuel/air mix 76. Additionally, the provision of a single relativelyhigh energy concentrated flame can damage the delicate porous wall ofthe burner requiring expensive replacement.

The present invention provides a pilot for igniting a combustible fluidstream, the pilot comprising an inlet for receiving a fuel gas into thepilot; means for igniting the fuel gas; and a generally annular conduitopen along an annular extent by a circumferential aperture for conveyingfuel gas from the inlet and so that when the fuel gas is ignited anannular pilot flame is generated.

Other preferred and/or optional aspects of the invention are defined inthe accompanying claims.

In order that the present invention may be well understood, embodimentsthereof, which are given by way of example only, will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 a shows a section through a pilot taken along line I-I in FIG. 2;

FIG. 1 b shows a section through a pilot taken along line J_J in FIG. 2;

FIG. 2 shows an elevation of the pilot indicating the lines of sectionI-I and J-J;

FIG. 3 shows an open flame burner comprising the pilot shown in FIG. 1;

FIG. 4 shows a radial burner comprising the pilot shown in FIG. 1;

FIG. 5 shows a further open flame burner comprising the pilot shown inFIG. 1;

FIG. 6 shows a prior art open flame burner;

FIG. 7 shows a prior art radial burner;

FIGS. 8 and 9 show a further pilot made integrally with a weirarrangement of a burner.

Referring to FIGS. 1 a, 1 b and 2, a pilot 10 is shown for igniting acombustible fluid stream. The pilot comprises an inlet 12, which maycomprise a venturi arrangement, for receiving a fuel gas 14, including afuel and air/oxygen mixture, into the pilot. Ignition means 16, forexample a spark igniter or an ignition electrode, are provided forigniting the fuel gas 14. A generally annular conduit 18 is open alongan annular extent forming a circumferential aperture 20. The conduit 18conveys fuel gas from the inlet to the circumferential aperture. Whenthe fuel gas flowing out of the circumferential aperture 20 is ignited acontinuous annular pilot flame is generated radially of the conduit.

The annular conduit as shown is circular, but may be formed of othershapes such as a square, rectangle or hexagonal. In this regard, theannular conduit conveys fuel gas from the inlet through 360° and allowsthe gas to flow from the inlet so that when it is ignited an annularflame is generated. Likewise, the annular flame generated need not bering-shaped, but instead corresponds generally to the shape of theconduit. For example, if the conduit is rectangular then a generallyrectangular flame is generated.

The conduit as shown in FIGS. 1 a, 1 b and 2 is open along its annularextent by the circumferential aperture 20. The aperture extends through360° to allow fuel gas to flow from the conduit approximately equallyabout its circumference. The aperture is determined in size to ensurethat an equal flow of fuel gas is distributed around the circumferencegenerating a consistent flame.

The fuel gas typically contains a mixture of fuel such as a hydrocarbonand oxygen or air.

As shown in FIGS. 1 and 2, the conduit 18 is open along a radially innerannular extent so that the circumferential aperture 20 faces radiallyinwardly. When the fuel gas 14 is ignited by the igniting means 16 theannular pilot flame is generated radially inwardly of the conduit forsurrounding a fluid stream to be ignited. In other arrangements, theconduit may be open in an axial direction, for instance, at the surfacefacing upwardly, or out of the page, in FIG. 2.

Although the pilot flame generated is annular only a single igniter 16is required since once the flame is ignited in the region of the igniterit readily spreads around the circumference of the pilot. Therefore, theignition means 16 as shown is disposed at a single location in thecircumference of the pilot and in use ignites the fuel gas to form thecontinuous annular pilot flame. The pilot flame may not be fully annularat all times after ignition as the flow of fuel gas may not bedistributed completely equally about the circumference or the fluidstream may occasionally extinguish part of the annular flame. However,generally in use the pilot is capable of generating a continuous annularpilot flame.

Monitoring or sensing means may be placed at intervals around the pilotfor monitoring or sensing characteristics of the pilot, such as thepresence of a flame, or the flow or pressure of fuel gas. Such sensingmeans may feed back to a control for the pilot which may be connected toa fuel gas supply valve or the igniting means.

The generally annular conduit 18 in FIGS. 1 a, 1 b and 2 comprises aplenum chamber 22 for receiving fuel gas from the inlet 14 and acircumferential channel 24 for channelling fuel gas from the plenumchamber to the circumferential aperture 20 for forming the continuousannular pilot flame. In use the plenum chamber 22 distributes the fuelgas generally equally around the annular extent of the conduit 18 at anequal pressure so that when ignited the continuous annular pilot flameburns generally evenly at approximately the same intensity and sizeabout the annular extent of the conduit. This arrangement promotes thegeneration of a pilot flame which is annular and unbroken.

The pressure of gas in the conduit 18 may be greatest in the regionproximate the inlet 12. Accordingly, the amount of gas exiting thecircumferential aperture 20 may be greatest near the inlet and hence theannular flame may be largest near the inlet 12. The plenum chamberarrangement as described above serves to distribute the gas equallyaround the conduit. Additionally it will be seen that inlet 12 is notaligned with channel portion 24. Instead gas entering through the inletmust flow along a tortuous path prior to exiting the pilot through thecircumferential aperture. In this way, the surface of the plenum chamberopposed to the inlet and the upper surface of the baffle as shown inFIGS. 1 a and 1 b act as baffles to the flow of fuel gas.

The pilot may be moulded from a metallic material in a single piece ortwo or more pieces which are subsequently fixed together. The channelportion 24, or other portion of the pilot which is in contact with orclosely adjacent to the pilot flame, may be formed from a heat or flameresistant material, such as a ceramic material or a metal such asstainless steel. It is preferable to avoid a metal surface in contactwith the pilot flame since this may generate undesirable nitrogen oxidecompounds.

In use, fuel gas is introduced to the conduit 22 and conveyed around theconduit so that it can flow out of the circumferential aperture 20. Whenthe flow through the aperture is established the ignition means 16ignites the fuel gas generating a pilot flame which extends radiallyinwardly from the aperture and in a generally annular configuration. Therate at which fuel gas is introduced to the conduit and the size of theaperture are selected to reduce the possibility of flash back into theplenum chamber 22. In this regard, the flame speed should be less thanforward velocity of the fuel gas. The flame speed is the speed a flamepasses back towards its source fuel and oxidant. The forward velocity isthe speed at which gas is conveyed through the aperture in a generallyradially inwards direction and should be greater in velocity than theflame speed. As the gas mixture leaves the aperture, it will spread out(fan like), thereby reducing the forward velocity. The anchor region atwhich the forward velocity equals flame speed is consequently justbeyond the exit of the aperture. If the material from which the mainportions of the pilot is made is metallic, flame resistant portions,made for instance of ceramic, may be disposed in the region of theaperture 20.

The pilot is formed in this example by a first, or lower, pilot plate116, which together with a second, or upper, pilot plate 117 forms theplenum chamber 22, the channel portion 22 and circumferential aperture20. The first and second pilot plates may be fixed together by anysuitable means such as by bolts. Spacers 121 are placed at intervalsaround the pilot plate for spacing the first pilot plate from the secondpilot plate. The spacers ensure that the channel portion 24 andcircumferential aperture 20 are sized correctly and uniformly around thepilot. Ignition means 16 which in this example is an ignition electrodeextends through the wall of the first pilot plate and is made gas tightby means of a swageing system. The ignition electrode extends into thecircumferential aperture to provide an ignition source when the fuel gasis flowing through the aperture. Inlet 12 is formed in the first pilotplate 116 which may be sealed with a Swagelok component.

An open flame burner 27 comprising pilot 10 is shown partially in FIG.3. The pilot 10 is not shown in detail in FIG. 3 for simplicity.

A fluid stream 29 is introduced through an inlet 31 of the combustionchamber 33 as a mixture with a fuel gas 35. The mixture is ignited by apilot flame 37 of the pilot 10 and burns as an open flame 39 combustingthe noxious substances. Although only a semi-circular portion of theflame 37 is shown in FIG. 3, the pilot flame 37 is annular and thereforesurrounds the fluid stream and fuel gas mixture as it enters thecombustion chamber 33 through the inlet 31. The term “horizontal pilot”as used herein is used to describe the arrangement shown in that thepilot extends generally perpendicularly to the direction of flow ofgases entering the burner. Usually a burner is upright as shown and inthis case the pilot is generally horizontal. Accordingly, the mixture isexposed to the pilot flame laterally from all sides ensuring relativelycomplete combustion of the mixture. This arrangement constitutes animprovement over the prior art described above in which only the portionof the fluid stream proximate the pilot flame is consistently burnt.Additionally, as the circumferential aperture 20 is relatively large andthe flame is therefore a distributed low energy flame the fuel gasflowing through the aperture does not significantly disrupt the mainflame 39. Referring to FIG. 4, a radial burner 26, comprising pilot 10,is shown for removing noxious substances from a fluid stream 28. Theburner 26 comprises a combustion region in which a fuel gas can be burntfor combusting the fluid stream. In this arrangement the combustionregion is formed by a chamber 30 surrounded by a generally cylindricalwall 32. The wall 32 is porous to allow passage of fuel gas through itinto the combustion chamber for burning on the inner surface of thewall. Fuel gas 34 is introduced to an outer chamber 36 through inlet 38and passes through the wall 32. The wall may form a right circularcylinder, elliptic cylinder, parabolic cylinder, or hyperbolic cylindersuch that the wall forms a surface on which fuel gas 34 can burnradiating hear radially inwardly and combusting the fluid stream. Itwill also be understood that the pilot 10 is located away from the top,or head, of the burner where space is limited.

A weir arrangement 44 produces a weir of cold liquid 46 (typicallywater) for dissolving constituents of the combusted fluid stream and forwashing away particulate matter. The cold liquid also cools fluidexhausted from the burner so that it can be conveniently disposed.

The pilot 10 is shown in simplified form in FIG. 4 and is located belowthe wall 32 and between the combustion chamber 30 and the weirarrangement 44. In use the pilot 10 generates an annular flame 40although only a semi-circular portion of which is shown in FIG. 4. Thepilot flame 40 ignites fuel gas at the inner surface of the wallproducing a flame 42. The fluid stream 28 containing at least onenoxious substance is introduced into the combustion chamber 30 throughinlet 48 and is combusted by contact with the hot reaction zone 42 nearthe surface of the wall 74.

The pilot 10 is located so that when ignited the continuous annularpilot flame 40 and the generally cylindrical wall 32 are adjacent alongtheir respective annular extents such that the fuel gas 34 passingthrough the generally cylindrical wall can be efficiently ignited and aflame at the surface maintained. As the annular pilot flame 40 ensuresthat the full circumferential extent of the lower portion of the flame42 is ignited and maintained alight, combustion of the fuel gas 34 overthe full inner surface of the wall 32 is increased.

Additionally, the pilot 10 is located so that when ignited thecontinuous annular pilot flame 40 thermally insulates the base of themain combustor reaction zone 42 from chilling effect generated by therelatively cold liquid 46 passing over the weir arrangement.Accordingly, heat is more efficiently generated at the base of thereaction zone 42, thereby improving the emissions from the combustor atthe base nearest the weir, for example carbon monoxide and hydrocarbonemissions (CxHy). Table 1 shows the improvement observed in testing theembodiment of the invention.

TABLE 1 Conditions Residual oxygen Carbon monoxide CxHy Prior Art shownin 5.2% 196 ppm 0.34% FIG. 7 Embodiment shown in 5.0% 100 ppm 0.17% FIG.4 with horizontal pilot

The improvement is due to a numbers of aspects of the embodiment. Forexample, heating the base of the radiant burner pad improves thecombustion at the base.

Powders tend to form at the base of the combustor due to reducedtemperature of the combustor pad 32/reaction zone at the base. Theprovision of a horizontal pilot thermally insulates the base of the padincreasing efficiency of the pad allowing it to sustain hottertemperatures. Additionally, the horizontal pilot decreases thepropensity for powder to adhere to the delicate porous structure of thepad 32. The problem of powder deposition still occurs but in theembodiment it occurs downstream of the pad 32. However, the pilot platesare relatively robust and the deposition of powder on the pilot is notconsidered a significant problem. The solids may include silica which iseasily removed by light agitation of the surface of the pilot bycompressed air or water flows. Further, the pilot can be cleaned bywashing with water in an in situ cleaning method (described in moredetail below with reference to FIGS. 8 and 9)

Advantageously, the pilot 10 provides a low energy distributed flamesurface which has less propensity to damage the delicate porous wall 32.

Although not depicted, the radial burner shown in FIG. 4 can be used incombination with the open flame burner shown in FIG. 3 for combustingcertain types of noxious substances in the fluid stream. In thisarrangement, the inlet upstream of the combustion chamber 30 shown inFIG. 4 is adapted for introducing the fluid stream and a fuel gas as amixture into the combustion region as shown in WO2006/013355 thecontents of which are hereby incorporated by reference. The mixture atthe inlet can be ignited by the flame 42 formed at the surface of thegenerally cylindrical wall which is itself ignited by pilot 10 at thebase of the cylindrical wall. Alternatively, the pilot 10 is locatedbetween the wall 32 and the inlet (at the top of the wall as shown inFIG. 4) for igniting both the mixture at the inlet and the fuel gas atthe surface of the wall.

FIG. 5 shows a modification of the open flame burner shown in FIG. 3.The burner 45 is suitable for removing noxious substances from aplurality of fluid streams 47, for instance, from a respective pluralityof semi-conductor wafer processing chambers. Burner 45 comprises aninlet 41, 43 for each fluid stream 47. Although only two inlets areshown the burner may comprise more than two inlets. The inlets 41, 43introduce the plurality of fluid streams 47 and fuel gas 49 asrespective mixtures into the combustion chamber 33. The pilot 10 islocated so that when ignited the continuous annular pilot flame 37surrounds mixtures to be burned so that all of the mixtures can beignited by a single pilot.

Typically, cleaning of the pilot 10 requires removal of the burnerarrangement 26 and this removal is labour intensive and results insignificant tool downtime.

However, in-situ cleaning of the pilot plate can be performed Cleaningis performed by pumping pressurized fluid, such as water or air, throughinlet 12 of the pilot, through the plenum chamber and out of thecircumferential aperture 20. Advantageously, tthe fluid is a liquid, asa liquid falls under gravity removing particulates from the outer wallof the lower pilot plate 116.

Further, the pilot plate 10 may be integrated with the weir 44. As shownin FIGS. 8 and 9, a passage 112 is machined between the pilot plateplenum 105 and the weir volume 107. A bung 106 is moveable from a firstposition in which it blocks the passage 112 and a second position inwhich the passage is open. When the passage is open, water injectedthrough inlet 12 of the pilot can flow into the pilot plenum 105 and outthrough the circumferential aperture 109, running down the entirety ofthe outer wall of the lower pilot plate 102, removing solid residue byphysical and chemical (dissolving) action. After cleaning the water isswitched off and the plenum chamber allowed to drain. Once drained, thebung 106 is then reengaged. The pilot is dried by blowing air throughthe inlet. Once dry, the pilot can be ignited. It will be appreciatedthat the process described for in-situ cleaning is more efficient andless time intensive than disassembling the burner, disconnecting variouspipes, cleaning the pilot and subsequently re-assembling the burner andre-connecting the pipes. Further, in-situ cleaning reduces thepossibility of the operator coming into contact with potentiallydangerous combustion byproducts (ie oxides of Arsenic on AsH3processes).

Accordingly, a method of cleaning pilot 10 in situ in a burnercomprises: disconnecting a source of fuel from an inlet to the pilot;connecting a source of cleaning fluid to the inlet; and cleaning thepilot with said fluid.

1. A pilot for igniting a combustible fluid stream, the pilot comprisingan inlet for receiving a fuel gas into the pilot; means for igniting thefuel gas; and a generally annular conduit open along an annular extentby a circumferential aperture for conveying fuel gas from the inlet andso that when the fuel gas is ignited an annular pilot flame isgenerated.
 2. A pilot as claimed in claim 1, wherein the conduit is openalong a radially inner annular extent so that when the fuel gas isignited an annular pilot flame is generated radially inwardly of theconduit.
 3. A pilot as claimed in claim 1 or 2, wherein when ignitedsaid pilot flame is continuous around the annular extent of the pilot.4. A pilot as claimed in any preceding claim, the ignition meanscomprises a spark igniter.
 5. A pilot as claimed in any preceding claim,wherein the ignition means comprises a single igniter and in use ignitesthe fuel gas to form the continuous annular pilot flame.
 6. A pilot asclaimed in any preceding claim, wherein the generally annular conduitcomprises a plenum chamber for receiving fuel gas from the inlet and acircumferential channel for channelling fuel gas from the plenum chamberfor forming the continuous annular pilot flame.
 7. A pilot as claimed inclaim 6, wherein in use the plenum chamber distributes the fuel gas at agenerally equal pressure around the annular extent of the conduit sothat when ignited the continuous annular pilot flame burns generallyevenly about the annular extent of the of the conduit.
 8. A burner forremoving noxious substances from a fluid stream, the burner comprising:a combustion region in which a fuel gas can be burnt for combusting thefluid stream; and a pilot as claimed in of claims 1 to 7 for ignitingthe fuel gas in the combustion chamber.
 9. A burner as claimed in claim8, wherein the combustion region is formed by a chamber having asurrounding generally cylindrical wall for introducing therethrough intothe combustion chamber a fuel gas for burning on a surface of said wallwhen ignited by the pilot so that a fluid stream passing through saidchamber can be combusted.
 10. A burner as claimed in claim 9, whereinthe pilot is located so that when ignited the annular pilot flame andthe wall are adjacent along their respective annular extents such thatthe fuel gas passing through the wall can be efficiently ignited and aflame at the surface maintained.
 11. A burner as claimed in claim 10,comprising a weir arrangement positioned downstream of the combustionchamber for receiving the combusted fuel stream, wherein the pilot islocated so that when ignited the annular pilot flame thermally insulatesthe generally cylindrical wall from relatively cold liquid passing overthe weir arrangement.
 12. A burner as claimed in any of claims 8 to 11,said combustion region comprising an inlet upstream of the combustionchamber for introducing the fluid stream and a fuel gas as a mixtureinto the combustion region.
 13. A burner as claimed in claim 12, whereinsaid mixture is ignited at said inlet by the flame formed at the surfaceof the generally cylindrical wall.
 14. A burner as claimed in claim 13,wherein said pilot is located between the generally cylindrical wall andthe inlet for igniting mixture at the inlet and the fuel gas at thesurface of the generally cylindrical wall.
 15. A burner as claimed inclaim 8, wherein said combustion region comprises an inlet upstream ofthe combustion chamber for introducing the fluid stream and a fuel gasas a mixture into the combustion region and said pilot is located sothat when ignited the annular pilot flame surrounds the mixture to beburned.
 16. A burner as claimed in claim 14, wherein comprising aplurality of inlets to the combustion chamber for introducing aplurality of fluid streams and fuel gas as respective mixtures into thecombustion chamber and said pilot is located so that when ignited theannular pilot flame surrounds mixtures to be burned so that all of themixtures can be ignited by said pilot.
 17. A burner as claimed in claim11 or 12, in which a portion of the weir arrangement is integral withthe pilot and wherein the pilot comprises a closeable passage which whenopen water used to clean the pilot flows through the passage and overthe weir, and when closed the pilot can form said annular flame.